Microfluidic platforms have attracted a great deal of attention in recent years and extensive research has been conducted in this area. They are a core technology used in a number of miniaturized systems that have been developed for chemical, biological, and medical applications. A wide range of microfluidic devices such as pumps, valves, mixers, and flow sensors has been demonstrated. In the past, these devices have been fabricated almost exclusively in silicon, glass, or quartz by applying techniques available in the microelectronics industry. For many applications, particularly in the life sciences (e.g. analytical chemistry, drug discovery, clinical diagnostics, etc.), these materials and the associated production methods are too expensive, or the material properties often induce problems such as a lack of optical clarity, low impact strength, and poor bio-compatibility. Polymers as substrate materials, on the other hand, provide a wide range of physical and chemical properties. They also have the advantages of low cost, good processibility for mass production, and are biocompatible and recyclable. Polymer microfabrication techniques have attracted great interest in recent years. Various methods (e.g. LIGA, UV-LIGA) and molding technologies (e.g. reactive casting, microembossing, and injection molding) have been applied to fabricate polymer-based microfluidic devices.
After fabrication, these devices need to be sealed in order to perform microfluidic functions. Packaging (i.e. sealing a device with a lid) is a challenging issue in the fabrication of polymer-based microfluidic devices. Bonding between silicon and silicon or other materials (e.g., glass, metal, etc.) is well developed and can be achieved by different methods such as anodic bonding, fusion bonding, eutectic bonding, and adhesive bonding. Among these techniques, only adhesive bonding can be applied to polymer-based microfluidic devices. When using the adhesive bonding method, care needs to be taken in order to prevent the adhesive from flowing into the micro channels. Other techniques such as adhesive tape bonding (lamination), thermal (IR, hot-plate, laser) bonding, ultrasonic welding, and solvent bonding (i.e., partially dissolve the bonding surfaces, and evaporate the solvent) have also been tried. Lum and Greenstein prepared microdepressions on one substrate and microprojections on the other so that the substrates can be mated together to secure the relative position. A layer of monomer or pre-polymer was deposited on the microprojections before being mated and further polymerized to provide a bonding effect. Dreuth and Heiden applied a thin adhesive film on a substrate and the adhesive was then transferred to the elevated microstructures by stamp printing. Glasgow et al. introduced a solvent bonding technique in which a layer of polyimide precursor and solvent with dissolved precursor was placed in contact with patterned structures made of uncured polyimide precursor. The two halves were then cured with weights on top of the upper plate. They found that the bond quality was affected by the vent spacing for solvent evaporation, soft-bake duration, spin-coat speed during solvent application, and the concentration of the dissolved polyimide precursor in the solvent. Basically, these approaches alter the surface of the microdevices by using external forces (e.g. solvent, adhesive, ultrasonic, laser) and applying pressure to bring two halves together. They have problems with either blocking the microchannels or changing their dimensions and are mainly applicable for relatively large microchannels (several hundreds of microns to millimeters).
In most bioMEMS applications, biocompatibility is one of the main requirements for the substrate used due to protein adsorption and cell adhesion on the substrate surface. For instance, blood thrombosis starts with deposition and aggregation of platelets on the surface of the prosthesis. In order to prevent thrombosis, the platelets should be kept from adhering to the surface of the prosthesis. Factors that affect protein adsorption include protein concentration, surface energy/tension, surface roughness, crystallinity, surface charge, etc. The surface free energy has been believed to be the dominant factor in protein adhesion. The thermodynamic driving force for protein deposition is low if the interfacial free energy between the substrate and the bio-fluids is low enough. It is believed that an interfacial free energy with water on the order of 1–3 mJ/M^2 could be a criterion for a material being biocompatible. Generally, the interfacial free energy between polymeric materials and bio-fluids is too large to meet the biocompatibility criterion. Therefore, surface modification is necessary to lower the interfacial free energy to produce surfaces that are able to resist cell adhesion and protein adsorption.
A great deal of effort has been devoted to the surface modification of biomaterials by coating the substrate surface with a hydrophilic material, such as polyethylene oxide (PEO) and polyethylene glycol (PEG), or with a surfactant such as sodium dodecyl sulfate (SDS), to increase the surface free energy of the biomaterials. It has been found that a PEG coating can change the surface from hydrophobic to hydrophilic, and thereby reducing protein adsorption and cell adhesion. The two-phase deposition of a surfactant (SDS) on a PMMA surface can also increase the hydrophilicity of a hydrophobic surface.
It is therefore a goal of the present invention to provide a method that can be used to bond microfluidic devices together. It is a further goal of the present invention to provide a method for general surface modification of microchannels in micro-fluidic devices. It is a further goal of the present invention to provide a method for providing localized surface modification of microchannels in micro-fluidic devices.